There are presently available, two general types of piston compression rings. One type uses a plane compression ring and the other type uses a wedge-shaped ring. Both types use one ring per groove on the piston. The rings are gapped in a manner to maintain ring clearance with dimensional changes of the cylinder bore with changing engine temperatures.
According to these standard ring arrangements described above the compression and gases blow-by and leak through the ring gaps into the crank case. This problem increases as the cylinder bore increases with wear because the rings move out to the increased bore size such that the ring gaps are continually opening further the more the engine is used providing an increased blow-by area. The blow-by of the compressed gases affects the power output of the engine and increases fuel comsumption. In addition there is increased air pollution.
In an attempt to overcome the blow-by problem the piston is normally provided with anywhere from two to four single compression rings which are initially set up in assembly such that the ring gaps are away from one another. However, as the engine is used, the gaps generally tend to align with one another due to the pressure of the escaping gases through the gaps attempting to escape through as short a route as possible into the crank case of the engine.
A further problem which has been encountered in deisel engines is that the conventional plane rings tend to seize in the top piston groove so that the wedge rings mentioned above were brought in to overcome this problem. However the wedge rings present their own problems because as the cylinder bore wears, the rings gradually move out of the piston groove to the cylinder wall allowing the ring to move abruptly up and down following the piston stroke, further increasing wear on the groove. This up and down movement of the wedge shaped ring eventually leads to the characteristic breaking of the top ring.